In particular, the brake disc is configured to be mounted on a hub of a wheel of the bicycle.
Preferably, said bicycle is a racing bicycle.
As known, it is now common in bicycles to use disc brakes. Such brakes are indeed often preferred to conventional brakes of a different type in that they ensure a high braking force and better modularity that allows a marked braking sensitivity, as well as being less subject to problems caused by mud or water.
Typically, a disc brake comprises a caliper fixed onto the frame of the bicycle and a brake disc mounted on the hub of the wheel. Inside the caliper there are two or four opposite brake pads. The brake disc rotates inside the space defined between the opposite pads. By actuating the brake lever, the pads are brought towards the brake disc, generating friction on the brake disc and, consequently, braking the wheel.
The brake disc comprises a braking track configured to cooperate with pads and a radially inner annular coupling portion for coupling with the hub.
The brake disc can be made in a single piece or in two components.
In this last case, the brake disc comprises a first component having the braking track and a second component having the radially inner annular coupling portion for coupling with the hub.
The first component is typically made of a first material that ensures good braking properties, like for example steel, whereas the second component is typically made of a second lighter material, like for example aluminum or light alloys.
The second component has a plurality of radially outer connection portions for connecting to the first component at a respective plurality of radially inner connection portions of the first component.
In the technical field, the second component is called “spider” or “carrier”, whereas the first component is called “rotor” or is simply indicated with the expression “braking track”.
The connection between the radially outer connection portions of the second component and the radially inner connection portions of the first component can be carried out so that the first component and the second component are substantially coplanar but not in direct contact, to avoid the occurrence of mechanical tensions in the radial direction due to different heat expansion coefficients of the two materials of the first and the second component. The two components of the brake disc are joined together by rivets or similar which also keep the rotor and the spider coplanar and slightly spaced apart in the radial direction. The brake disc with this type of coupling, known as floating rotor brake disc, has the drawback of having potential twisting, since the planarity of the brake disc is, basically, entrusted only to the rivers.
Alternatively, the two components of the brake disc are coupled together so that regions between portions of the rotor and portions of the spider overlap, making a so-called “non floating rotor”. Also in this case, rivets or similar stably join the two components of the brake disc, but the structural continuity of the entire brake disc is also ensured by the overlapping region between rotor and spider.
As far as the coupling with the hub is concerned, the radially inner annular portion of the brake disc is provided with a grooved radially inner surface (in other words a radially inner surface that extends longitudinally and is provided with longitudinal grooves), which is mounted on a matching grooved radially outer surface (in other words a radially outer surface that extends longitudinally and is provided with longitudinal grooves matching those of the radially inner surface) of a portion of the hub, as disclosed in patent application EP 1932753 to the same Applicant.
EP 1932753 also discloses a lock nut, which is screwed onto the hub until it abuts axially on the radially inner annular portion of the brake disc, so as to define a stable axial position of the brake disc on the hub.
The radially outer connection portions of the second component typically consist of a plurality of arms arranged equally circumferentially spaced and extending from the radially inner annular coupling portion. Such arms can have a substantially radial arrangement or, preferably, are inclined with respect to the radial direction, typically with the same inclination for every arm. The number of arms is variable: for example brake discs with five or six arms are well known.
The aforementioned arms are typically made in one piece with the radially inner annular coupling portion.
The braking force exerted by the pads of the brake on the braking track is transmitted to the radially inner annular coupling portion of the brake disc and discharged on the hub of the wheel. Therefore, the braking force exerted by the pads of the brake creates a torque at the radially inner annular coupling portion of the brake disc that is compensated by a pair of resistant forces of equal intensity and opposite direction at the coupling between brake disc and hub of the wheel.
The Applicant has observed that at the connection between arm and radially inner annular coupling portion there are areas with high concentration of stresses, since in these areas the distance from the pad of the brake is substantially maximum and, therefore, the torque transmitted is also maximum. For this reason, as stated above, the cited arms are preferably inclined with respect to the perfectly radial direction, to try to decrease the component of the braking force perpendicular to the arm itself (and therefore the torque transferred to the radially inner annular coupling portion).
The Applicant has however noted that the inclination of the arms with respect to the radial direction cannot be increased beyond a certain limit, since the arms must effectively reach the radially inner annular coupling portion.
The Applicant has also noted that increasing the inclination of the arms and, at the same time, ensuring that they effectively reach the radially inner annular coupling portion means an increase in the size of the arms, with consequent increase in the weight of the brake disc.
The Applicant has perceived that it would be advantageous to ensure an effective and safe transmission of forces between braking track and radially inner annular coupling portion, at the same time keeping down the weight of the brake disc.